X-ray Transient Localization Experiment aboard a micro-satellite formulti-messenger counterpart search
Tatsuya Sawano,1 Daisuke Yonetoku,1 Kazuki Yoshida,1 Yasuaki Kagawa,1Kentaro Kawai,1 Masao Ina,1 Tatehiro Mihara,2 and Hirokazu Ikeda3
1Kanazawa Univ., Kakuma-machi, Kanazawa, Ishikawa, 920-1192, Japan
2RIKEN, 2-1, Hirosawa, Wako, Saitama 351-0198, Japan
3ISAS/JAXA, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa, 252-5210, Japan
E-mail(TS): sawano@ astro.s.
.s.kanazawa-u.ac.jpAbstract
Short gamma-ray bursts (short GRBs) are likely caused by the coalescence of double neutron star binary
and/or neutron star and black hole binary, and therefore they are expected to be promising electromagnetic
counterparts of gravitational wave (GW) sources. The joint observation of short GRB and GW, and theidentification of the host galaxy and distance of such events will provide not only the definitive observational
evidence for the origins of short GRBs but also the insights to the compact object merger model and theexplosion mechanism. However, GW sky location is uncertain by tens or hundreds of square degrees, which
is not well constrained for the efficient follow-up observation with optical/NIR telescopes. Thus, we aredeveloping X-ray imaging detector that has both fine localization accuracy and wide field-of- view, named
Transient Localization Experiment (T-LEX). The energy range, field-of- view, localization accuracy of TLEX are designed to be 1 - 15 keV, 1 steradian, and 15 arcminutes, respectively. To achieve such capability,
we adopted two sets of 1-dimensional coded aperture imaging system, which consists of tungsten maskwith random pattern of 1-dimensional strip-like apertures and silicon strip detector with an active area of50 cm2 for each dimension. T-LEX is plan to be launched onboard a micro-satellite in late FY 2018, which
is being developed at Kanazawa University named as Kanazawa-SAT 3 . The mission life is designed tobe more than 1 year. The objective of the satellite mission is to localize X-ray transients including GW
sources and to send real time alerts of X-ray transient information about the position and the burst trigger
time. We report the imaging performance of a prototype model of T-LEX.
Key words: instrumentation: detectors — micro-satellite - gamma-ray bursts — coded mask imager

1. Introduction
The progenitors and the environment of short duration
gamma-ray bursts (GRBs) (< 2 sec) remain elusive despite of their localizations. To test the theoretical argument that short GRBs are produced in the coalescenceof binary compact objects such as neutron stars or black
holes (Paczynski 1986; Eichler et al. 1989), the observation of short GRBs with gravitational wave (GW) connection gives a decisive evidence.
Recently, the first direct GW detection, GW 150914,
was performed by advanced LIGO (Abbott et al. 2016a),and therefore the progress of gravitational astronomy ismuch expected in the future. To maximize the scientificreturn from the GW observation, it is essential to observe electromagnetic counterparts and identify the hostgalaxies of GW sources. However, the GW detectors
cannot determine the host galaxy of the GW events bythemselves due to their marginal localization ability. For
example, the constraint on the position of GW 150914
was an area of about 600 square degrees (Abbott et al.
2016b). Therefore, the observation of short GRBs witha wide field and a fine localization will play a crucial role
to serve a bridge from the GW detection to the electromagnetic follow-up observation.
2. Micro-satellite Kanazawa-SAT 3
Kanazawa-SAT 3 is a micro-satellite with a size of 50
cm cube weighing 50 kg promoted by Kanazawa University, and possesses a wide-field X-ray imaging detector(Yonetoku et al. 2014a). The objectives of that satellite mission are to monitor X-ray transients associatedwith GW events, and to provide the position and theburst trigger time of X-ray transients to the groundbased telescope community. Thus, Kanazawa-SAT 3 is

T-LEX. The energy range of T-LEX is designed to be
1 - 15 keV. The field-of-view is more than 1 sr so as tocover the constraint region of GW events efficiently. The
localization accuracy is about 15 arcminutes, which iswell constrained for the follow-up observation with optical/NIR telescopes. The effective area of the X-ray
sensors for X-/Y- planes is about 100 cm2 in total.
Table 2. Configuration of T-LEXFig. 1. A schematic view of Kanazawa-SAT 3 designed by Meisei
Electric Co., Ltd.

dedicated to contributing to the identification of theelectromagnetic counterparts of GW events. Utilizing
the advantage of short development time for a microsatellite, Kanazawa-SAT 3 is planned to be launched inlate FY 2018, in which the global network of the second generation GW interferometers will be in operation.
Table 1 summarizes configurations of Kanazawa-SAT 3 .
The mission life of the satellite is designed for at least one
year, and three years are set as the operational goal. The
orbit of the satellite is supposed to be a Sun-synchronous
orbit due to multiple launch opportunities.
Table 1. Configuration of Kanazawa-SAT 3

Mission lifeSize
WeightLaunch target
Orbit
Detector

Imaging method
Energy range
Field of view
Localization accuracy
Detector size
Mask aperture fraction

3.2. Design of Coded Aperture System
The coded aperture system of T-LEX consists of a tungsten mask with random pattern aperture and a silicon
strip detector (SSD) with a thickness of 0.5 mm. The
pitches of the SSD and the mask pattern are the same,and 0.3 mm. Generally, the localization pitch of thecoded aperture system for the line-of-sight δθ yields
dδθ = arctan
D> 1 year
3 years (goal)
50 cm cube
< 50 kgin late FY 2018
Sun-synchronouswide-field X-ray imaging detector

X-/Y- 1-D coded mask
1 - 15 keV> 1 sr∼ 15 arcmin. (geometrical)
100 cm2 in total (X and Y)
0.5

,

(1)

where d is the pitch of the detector and the mask pattern,
and D is the distance between the mask and detector
planes. Therefore, in order to satisfy the requirementof the localization accuracy, the distance D is set to beabout 70 mm.
3Figure 1 shows a schematic view of Kanazawa-SAT .
The satellite has two solar paddles, and a three-axis attitude control system for sun-oriented operation. The
X-ray imaging detector, T-LEX (Transient LocalizationExperiment) is installed on the opposite side of the panel
on which the solar panel is mounted, for optical light andthermal protections. Therefore, T-LEX will keep pointing at the anti-solar direction, and can monitor and observe the sky on the night side of the Earth.
3. Transient Localization Experiment (T-LEX)
3.1. Detector Overview
T-LEX consists of two (X and Y) sets of 1-dimensional
coded aperture systems. Table 2 shows configurations of

3.3. Readout Electronics
The charge signals of X-ray events from SSDs are verysmall (∼ 0.04 fC for 1 keV) and therefore they must be
amplified in order to discriminate with a proper energy
threshold. However, the number of the channels of theSSD is 512 (X) plus 512 (Y), and one cannot implement
the readout system with discrete circuits. Therefore,
we developed an application specific integrated circuit(ASIC) for readout of small charge from an X-ray sensor, ALEX (ASIC for Low-Energy X-rays) (Yonetoku etal. 2014b). ALEX has 64 analog inputs and digital input/output serial interfaces. The electrodes of ALEX
inputs and the SSDs are directly connected with bonding wires for least additional stray capacitance. Eachanalog input of the ALEX is processed to a chargesensitive amplifier, two shaping amplifiers with slow andsame. We made a case of the prototype of T-LEX madeof aluminum, where (d/D)−1 is designed to be 2.24×102.

Fig. 2. A picture of the sensor boards stacking on the readout boardsof the T-LEX prototype model.

fast time constants of several microseconds, and a comparator from the fast shaping amplifier output, and ananalog-digital converter from the slow shaping amplifier.
A field programmable gate array (FPGA) is used tocontrol the ALEX and process the event data throughthe digital interfaces of the ALEX. In this way, eightASICs are used to readout the X- (or Y-) sensor andthose ASICs are controlled with one FPGA. To control
two (X- and Y- ) FPGAs, another FPGA is used formerging the X- and Y- event data and communication
with the CPU on the satellite bus system. In the backend readout FPGA, the logic of one CPU core is implemented so as to manage complicated procedures suchas a burst trigger system and the serial communication
with the bus system.
3.4. Prototype Model
We developed a prototype model of T-LEX. A pictureof the sensor boards stacking on the readout boards ofthe T-LEX prototype model is shown in Figure 2. The
SSD installed on the sensor board in the prototype modelcomprises 512 strips for each (X- and Y-) dimension, andeach strip size is 0.3 × 16 mm2 . Therefore, the detector
area of the SSD for each dimension is ∼ 25 cm2 , whichis the half of the full configuration at the flight model.
On the other hand, the number of the readout channels is the same as that of the flight model, and thus
this prototype model can be characterized as the engineering model of the T-LEX electronics. There are eight
ASICs on each (X- and Y-) sensor board, and the FPGA
is mounted on the readout board on the back of the sensor board. The FPGA with the implemented CPU isalso mounted on the X- readout board. To reduce thechanges in the geometrical and functional designs for theflight model, the almost of the all electronics are pincompatible devices with the space-grade ones.
As a trial piece of the random pattern mask, we madea small tungsten mask with a thickness of 35 microns.
The aperture size of that tungsten mask is 76.8 × 76.8mm2 for one dimension, which is a quarter of the flightmodel. We note that we cannot perform the full-coded
imaging test using the all channels of the SSDs because
the widths of the sensor and the mask aperture are the

4. Imaging Performance of T-LEX Prototype Model
4.1. Overview
To confirm the validity of the basic design of T-LEX,
we performed an imaging demonstration test with partial channels of the SSDs. The goal of that test is asfollows. First, we measure the reconstructed directionsof X-ray beams with various incident angles. Second, wecompare those experimental data with the ones obtained
geometrically. Last, whether the discrepancy of them is
acceptable or not is discussed.
4.2. Reconstruction method
To obtain the reconstructed image of the coded masksystem, we used a cross correlation method expressed
withX
di · mi+j ,
(2)
rj =i

where the j is the shift index in the cross correlation
calculation, rj is the intensity at the shift index of jin the reconstructed image, di is the number of the Xray events at the detector position of i, and mi+j is themask weighing design element. In this paper, we usedthe condition on the mask weighing where mi is 0 if themask is opaque at i and 1 if the mask is transparent.
4.3. Setup of Experiment
As the parallel light of X-ray photons, we used a
5.5-meter beam line at the astrophysics laboratory ofKanazawa University. X-ray photons from an X-ray generator with a tube voltage of 40 kV irradiate the molybdenum target with a size of 5 × 5 cm2 so as to produce fluorescent X-rays with an energy of 17.5 keV (Kαline). The T-LEX prototype model is placed in a thermostatic bath, which is at a distance of 5.5 meters fromthe molybdenum target with an ambient temperature of−10 degrees Celsius. A rotation stage is installed on
the side of the case of the T-LEX prototype model inorder to change the incident angle of the X-ray beam.
The divergenceof the X-ray beam is calculated to be√(5 cm/ 2)/(5.5 m) ≈ 0.0064, or 22 arcminutes, and it isslightly larger than the localization accuracy of T-LEX.
This effect causes the peak dilation in the reconstructedimage.
As mentioned in Section 3.4., we cannot use the wholechannels under the full-coded condition. Thus, we usedthe data from two ASICs, or 128 channels that are close
to the center. Using those 128 channels of X-ray intensity
data and 256 strips of the mask patterns, we obtained thereconstructed image according to Eq. 2. We obtained
the peak position of the reconstructed image by fitting

4.4. Result and Discussion
Figure 3 and 4 show examples of di and rj , respectively,
obtained with the experiment. Figure 5 shows the peak
position in terms of the shift index versus the X-ray incident angle (or the rotation angle), where the origin isarbitrary. Ideally, the curve can be modeled with a tangent function because the peak position m yields
md,
D

model
measured5

3.5×105

3.0×105

2.5×105

2.0×105

1.5×105

1.0×104

5.0×10050

100
150
Shift Index

200250(3)Fig. 4. An example of the one-dimensional reconstructed image
within the half-coded region (red solid line). The model image
for parallel light is also drawn (gray dashed line).
280

model fit

230
Shift Index

where θ is the rotation angle from the line-of-sight. We
fitted this curve with a tangent function and obtained(d/D)−1 to be 2.27 × 102 using Eq. 3, which is 1.5%
larger than that designed. That means if we use thedesigned value for (d/D)−1 , the reconstructed angle forthe events with an incident angle of 30 degrees has anerror of about 30 arcminutes, which is not acceptable dueto the requirement. Therefore the calibration of (d/D)−1
by the imaging test will work effective to compensate
that systematic error. It is suggested that the imaging
test will be mandatory also for the flight model.

180

Residualtan θ =54.0×10
Correlation Intensity (arb. unit)

the data around the peak with a normal distributionfunction.

800.30-0.3

130-20-1001020

Incident Angle (degrees)Counts (arb. unit)

6000

Fig. 5. The peak positions in terms of the shift index in reconstructed
image versus the incident angles obtained by the experiment (redcross). The coordinate of the origin is arbitrary. The best estimateof the tangent model function is drawn in the blue solid line. The
residuals are also shown.

4000

2000

080

100120140
Strip Number

160180Fig. 3. An example of the X-ray intensity distribution (yellow-filledhistogram) representing the mask pattern obtained by the X-ray
beam irradiation. Behind the histogram, the best matching position of the mask is shown, where the transparent elements of themask are represented with the gray color region.

5. Summary
The X-ray transient localization with a wide field-of-view
will strongly promote the electromagnetic follow-up observation for the identification of the host galaxy of GW
events, contributing to the progress of GW astronomy.
The micro-satellite Kanazawa-SAT 3 possesses the widefield X-ray imaging detector, T-LEX, and the objectivesof that satellite mission are to monitor the X-ray transients associated with GW events and to alert to theground-based telescope community sending the information of the position and the burst trigger time of X-raytransients.
We developed the prototype model of T-LEX and successfully performed the imaging test. We found that theimaging performance test effectively works as the calibration for reconstructing the incident angles. That calibration will be also performed carefully to the flight modelof T-LEX to satisfy the requirement for the localization
accuracy.
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